Place water, onions, and pepper in the cooker. In a small bowl, combine barbecue sauce and mustard.

Set a layer of the largest ribs on top of the vegetables and slather with sauce. Repeat layering and slathering with remaining ribs. Pouring any extra sauce on tip.

Nestle lid in place by lining up marks on cooker handle and lid. Turn lid to align handles, then lock by push regulator into "zu/locked" position. Set over high heat. Just before 2nd orange ring on cooking indicator is visible, lower heat to maintain pressure at 2nd orange ring. Set timer for 25 minutes.

When timer rings, release pressure by gradually moving regulator toward end of long handle. When all pressure has been released, turn handle to right until marks are lined up, then lift lid by titling it away from you.

Transfer ribs to large serving platter and cover with foil to keep warm. Skim off surface fat. Transfer 2 tablespoons of sauce into a small bowl and blend with cornstarch.

Pour this slurry into sauce and stir while cooking at a gentle boil until sauce thickens, 2 - 3 minutes. Pour sauce over ribs or serve in a sauce boat.